Windows are areas where the skull bones are relatively thin or where the naturally occurring forage allows proper penetration of the ultrasound beam. These windows are commonly used for transcranial Doppler (TCD) examinations. However, in the best of cases, only approximately 6% of the intensity of the US used reaches the brain substance.

Different usual windows:

The acoustic window or field is the area defined by the pathway of the ultrasound beam between the transducer and the acoustic reflector. The sound reflection to skin boundary should be minimized with an ultrasound gel where this gel acts as an acoustic window through which the image is seen.
Acoustic window refers also to the optimal placing of the transducers so that the areas of interest are clearly imaged.

See also Transforaminal Window, Transcranial Window, Transorbital Window and Transtemporal Window.

The first step in a transcranial Doppler (TCD) examination is to localize a cranial acoustic window where the ultrasound beam can penetrate without being excessively attenuated.

There are three main transcranial acoustic windows, used for the sound beam in cerebrovascular ultrasonography to overcome the skull barrier:
A complete TCD examination incorporates these windows allowing the visualization of the complete cerebral circulation.

The transorbital window allow to insonate ophthalmic artery and ipsilateral carotid siphon through the eye. It is also possible to use the transorbital entrance to scan the middle and anterior cerebral arteries, if it is impossible through the transtemporal window.
The patient is asked to remove any contact lenses prior to examination and is instructed to close the eyes. The power output of the Doppler instrument has to be decreased to 10-20% to reduce the ultrasonic exposure of the eye.

See also Transcranial Doppler, Ultrasound Safety.

The temporal area is the thinnest portion of the skull and the squamous component with less cancellous bone provides ultrasound permeability. The transtemporal window is found between the angle of the eye and the pinna of the ear above the zygomatic ridge. Finding this window can be difficult because size and location vary with each patient (more difficult in elderly and females) and from one side to the other.
This window allows the insonation of the middle, anterior and posterior cerebral arteries, the anterior and posterior communicating, and the terminal internal carotid.

See also Transcranial Doppler.